1. Technical Field
The present invention relates to an ultrasound diagnostic apparatus, and, in particular, to a technique for forming an ultrasound beam using an array transducer.
2. Background Art
Two-dimensional array transducers constructed by two-dimensionally arranging a plurality of transducer elements are known. The two-dimensional array transducer is formed, for example, with a few thousand transducer elements which are electrically controlled. With the two-dimensional array transducer, ultrasound beams are two-dimensionally scanned, and echo data are three-dimensionally collected.
When a plurality of transducer elements which are a part of the two-dimensional array transducer are controlled, if a signal line is provided independently for each transducer element, a number of signal lines equaling the total number of transducer elements; for example, a few thousand signal lines, would be required for the overall two-dimensional array transducer. When a few thousand signal lines are employed, a probe cable connecting a probe in which the transducer elements are stored and a device body would become thick, and the number of pins of the connector in a connection portion between the probe cable and the device body also becomes large. The increase in circuit size for transmission/reception systems cannot be ignored. In consideration of these circumstances, techniques have been proposed for reducing the number of signal lines (number of channels) connecting the plurality of transducer elements and the device body.
For example, Patent Literature 1 (JP 2008-514335A) discloses a technique in which the two-dimensional array transducer is separated into a plurality of sub-arrays, reception signals of, for example, nine (three in vertical×three in lateral) transducer elements in each sub-array are delay-added, to sum signal of each sub-array into one signal at the side of the probe, and, then, the one signal line is connected to the device body. With such a structure, the number of channels can be significantly reduced to, for example, 1/9.
In order to sum the sub-array signals into one signal, information of amount of delay for each of, for example, nine transducer elements belonging to the sub-array is required, and the information is supplied from the device body to the probe. When the amount of delay is adjusted according to a steering angle of the beam and a depth of focus, the amount of the information to be supplied from the device body to the probe becomes very high. For example, when the amounts of delay for a few thousand transducer elements are transferred, a transfer time of a few tens of microseconds would be required, which may result in other problems, such as reduction in the frame rate.
Patent Literature 2 (JP 2000-33087 A) discloses a technique to apply a control such that the depth of focus is fixed at infinity and only the beam steering is considered, to simplify the control of delay and reduce the amount of information. However, when the depth of the focus is simply set to infinity, converging of the beam etc. may be degraded and the precision of the beamforming is reduced, resulting in reduction in the resolution or the like of the image.
Under such circumstances, the present inventor have researched and developed a technique which maintains the precision of the beamforming while inhibiting increase in the amount of information of the delay process.